Senin, 22 Juni 2009

Cesium vapor magnetometerA basic example of the workings of a magnetometer may be given by discussing the common "optically pumped cesium vapor magnetometer" which is a highly sensitive (0.004 nT/√Hz) and accurate device used in a wide range of applications. Although it relies on some interesting quantum mechanics to operate, its basic principles are easily explained.

The device broadly consists of a photon emitter containing a cesium light emitter or lamp, an absorption chamber containing cesium vapor and a "buffer gas" through which the emitted photons pass, and a photon detector, arranged in that order.

PolarizationThe basic principle that allows the device to operate is the fact that a cesium atom can exist in any of nine energy levels, which is the placement of electronatomic orbitals around the atomic nucleus. When a cesium atom within the chamber encounters a photon from the lamp, it jumps to a higher energy state and then re-emits a photon and falls to an indeterminate lower energy state. The cesium atom is 'sensitive' to the photons from the lamp in three of its nine energy states, and therefore eventually, assuming a closed system, all the atoms will fall into a state in which all the photons from the lamp will pass through unhindered and be measured by the photon detector. At this point the sample (or population) is said to be polarized and ready for measurement to take place. This process is done continuously during operation.

DetectionGiven that this theoretically perfect magnetometer is now functional, it can now begin to make measurements.

In the most common type of cesium magnetometer, a very small AC magnetic field is applied to the cell. Since the difference in the energy levels of the electrons is determined by the external magnetic field, there is a frequency at which this small AC field will cause the electrons to change states. In this new state, the electron will once again be able to absorb a photon of light. This causes a signal on a photo detector that measures the light passing through the cell. The associated electronics uses this fact to create a signal exactly at the frequency which corresponds to the external field.

Another type of cesium magnetometer modulates the light applied to the cell. This is referred a Bell–Bloom magnetometer after the two scientists who first investigated the effect. If the light is turned on and off at the frequency corresponding to the Earth's field, there is a change in the signal seen at the photo detector. Again, the associated electronics uses this to create a signal exactly at the frequency which corresponds to the external field.

Both methods lead to high performance magnetometers.

ApplicationsThe cesium magnetometer is typically used where a higher performance magnetometer than the proton magnetometer is needed. In archaeology and geophysics, where the sensor is moved through an area and many accurate magnetic field measurements are needed, the cesium magnetometer has advantages over the proton magnetometer.

The cesium magnetometer's faster measurement rate allow the sensor to be moved through the area more quickly for a given number of data points.

The lower noise of the cesium magnetometer allows those measurements to more accurately show the variations in the field with position.

The SERF magnetometers only operate in small magnetic fields. The Earth's field is about 50 µT. SERF magnetometers operate in fields less than 0.5 µT.

As shown in large volume detectors have achieved 200 aT/√Hz sensitivity. This technology has greater sensitivity per unit volume than SQUID detectors.

The technology can also produce very small magnetometers that may in the future replace coils for detecting changing magnetic fields.

Rapid developments are ongoing in this area. This technology may produce a magnetic sensor that has all of its input and output signals in the form of light on fiberoptic cables. This would allow the magnetic measurement to be made in places where high electrical voltages exist.

SQUID magnetometer

SQUIDs, or superconducting quantum interference devices, measure extremely small magnetic fields; they are very sensitive vector magnetometers, with noise levels as low as 3 fT·Hz−0.5 in commercial instruments and 0.4 fT·Hz−0.5 in experimental devices. Until the advent of SERF atomic magnetometers in 2002, this level of sensitivity was unreachable otherwise.These magnetometers require cooling with liquid helium (4.2 K) or liquid nitrogen (77 K) to operate, hence the packaging requirements to use them are rather stringent both from a thermal-mechanical as well as magnetic standpoint. SQUID magnetometers allow one to measure the magnetic fields produced by brain or heart activity (magnetoencephalography and magnetocardiography, respectively).

Early magnetometersIn 1833 Carl Friedrich Gauss, head of the Geomagnetic Observatory in Göttingen, published a paper on measurement of the Earth's magnetic field. [10] It described a new instrument that Gauss called a "magnometer" (a term which is still occasionally used instead of magnetometer). It consisted of a permanent bar magnet suspended horizontally from a gold fibre. A magnetometer is also called a gaussmeter.